CALICE 03/14/05Ed Norbeck U. of Iowa1 PPACs in a Calorimeter Edwin Norbeck University of Iowa.

CALICE 03/14/05 Ed Norbeck U. of Iowa 1

PPACs in a Calorimeter

Edwin Norbeck

University of Iowa

CALICE 03/14/05 Ed Norbeck U. of Iowa 2

What is a PPAC?

•Two flat, conducting plates with a little gas between them

•Simple, low cost device

•Can be radiation hard

•Unaffected by heat, light

•No electronics or photodetectors attached

(Parallel Plate Avalanche Counter)

CALICE 03/14/05 Ed Norbeck U. of Iowa 3

One plate can be divided into pixels to provide position resolution.We have studied a small PPAC as a single pixel of a detector for electromagnetic showers.

What we learned from these studies have broad application.

•Results from small PPAC

•Use of PPACs in calorimeters

Outline of talk:

CALICE 03/14/05 Ed Norbeck U. of Iowa 4

Single Pixel PPAC For Test With High-Energy Electrons

• Gap 0.6 mm 950 V across gap• Cathode 7X0 = 29 mm of tantalum• Area of anode is 1.0 cm2

• Guard ring to simulate neighboring pixels

• Gas is isobutane at 120 torr

CALICE 03/14/05 Ed Norbeck U. of Iowa 5

Detail of gap and guard ring

CALICE 03/14/05 Ed Norbeck U. of Iowa 6

Test at home with a 7 mCi 137Cs source

Get up to 20 mV signals directly into 50 coax

CALICE 03/14/05 Ed Norbeck U. of Iowa 7

Signal into coax with no amplifier

Signal observed directly with fast scope

CALICE 03/14/05 Ed Norbeck U. of Iowa 8

We did not have high-energy electrons so we made them in situ from protons interacting near the front end of our tantalum cylinder.

The showers had amplitudes as much as 40 mV

CALICE 03/14/05 Ed Norbeck U. of Iowa 9

Signal shape from shower

-30 mV

1.62 ns FWHM

CALICE 03/14/05 Ed Norbeck U. of Iowa 10

•The signal comes from moving charges.

•In an avalanche, most of the electrons and ions are formed near the anode.

•The electron signal is fast but with a total area small compared with the ion signal.

•The ion signal is flat while the ions are moving and stops when the ions are collected.

•In the next slide, when the ions are collected at the cathode they liberate electrons.

CALICE 03/14/05 Ed Norbeck U. of Iowa 11

20 torr ethane 550 V 0.6 mm gap

CALICE 03/14/05 Ed Norbeck U. of Iowa 12

PPAC gassesAlkanes [methane (CH4), ethane (C2H6), butane, etc.] good Perfluoro analogs (CF4, C2F6, etc.) even better

We have used:

CF4 (AKA carbon tetrafluoride and Freon 14)

C3F8 (Perfluoropropane)

C4F8 (Perfluorocyclobutane)

CALICE 03/14/05 Ed Norbeck U. of Iowa 13

Advantages of Perfluoro gassesNot toxic

Does not burn

Resists aging

Easily recycled (A small chemical cartridge can be inserted in the gas line that will remove all impurities from the gas)

High density (more primary electrons)

Contains no chlorine so is allowed in national laboratories

CALICE 03/14/05 Ed Norbeck U. of Iowa 14

CF4 at one atmosphere

Required 2500 V with plate spacing of 0.6 mm

Less than 1.5 ns electron signal

Ion collection time 960 ns

PPAC design much simplified at one atm.

CALICE 03/14/05 Ed Norbeck U. of Iowa 15

Beam In

Typical Calorimeter

The green is solid metal (W). Detectors that sample the shower

are shown in blue.Detector near front end is for EM

shower

CALICE 03/14/05 Ed Norbeck U. of Iowa 16

For electromagnetic showers in a high Z material the final deposition of most of the energy is by low energy electrons.

If the plates of a PPAC are made of the same high Z material, the PPAC will provide a faithful sample of the energy deposition in the absorber. This does not work for thicker detectors because the lower energy electrons stop in the surface of the detector.

The sampling fraction in a PPAC is small. The fractional error from sampling fluctuations is proportional to E-½. Fluctuations are not a problem if the showers have a large enough energy.

CALICE 03/14/05 Ed Norbeck U. of Iowa 17

High-energy showers with double PPAC

•Test with EM showers using 80 ps bunches of 7 GeV positrons from the Advanced Photon Source, at Argonne National Laboratory

•Each bunch contained 3.6 x 1010 positrons

•The showers were made by the beam halo striking the beam pipe. The energy was a small fraction of the 2 x 1020 eV in the bunch

•This is still a very large energy!

(Shower passes through both PPACs)

CALICE 03/14/05 Ed Norbeck U. of Iowa 18

Double PPAC for testing energy resolution

CALICE 03/14/05 Ed Norbeck U. of Iowa 19

Energy Resolution Data of PPAC Test at ANL

Ratio Efront to Eback is constant to within ± 2%

CALICE 03/14/05 Ed Norbeck U. of Iowa 20

CONCLUSIONS

PPACs for a calorimeter•Can be made radiation hard.

•Can provide position information.

•Have good energy resolution for high energy showers.

•Have sub nanosecond time resolution.

•Can connect PPACs directly into 50 coax

•Can test with source on side

•Can operate at atmospheric pressure with CF4 gas

CALICE 03/14/05 Ed Norbeck U. of Iowa 21

CALICE 03/14/05 Ed Norbeck U. of Iowa 22

At isobutane pressures less than 30 torr afterpulses sometimes occur during the first 20 ns.

This is a worst case example.

Total charge from the afterpulses can be much larger than primary signal.

10 torr 500 V